Conventionally, a printed circuit board is manufactured as follows: while a printed wiring board bearing a circuit pattern is transported, a paste, such as a cream solder and a conductive paste, is first printed by a printer on the printed wiring board at points onto which components are to be mounted, and then components are successively mounted onto the printed points by a surface mounting apparatus (hereinafter, referred to simply as mounting apparatus).
The printer is configured to superimpose a mask sheet on a printed wiring board set, for example, on the stage and to press paste supplied on the mask sheet using a squeegee, so that paste is printed on the printed wiring board at predetermined positions via openings formed in the mask sheet. After the printing, an image of the printed wiring board is taken by a print inspection apparatus incorporated in the printer or a print inspection apparatus disposed downstream of the printer, and the print state is inspected on the basis of the image.
The print inspection apparatus determines whether the product is acceptable or unacceptable by measuring a quantity in connection with an amount of paste printed on the printed wiring board to be inspected, for example, an area of printed paste, and then determining whether the measurement value falls within a reference range set in advice according to designed values of the mask sheet or the like.
Also, as is disclosed in Japanese Patent Unexamined Publication No. HEI6-27033, not only to determine whether the product is acceptable or unacceptable, but also to determine a tendency (smearing tendency and fainting tendency) of print state even when measurement value falls within the range of an acceptable product, an absolute reference value used to determine whether the product is acceptable or unacceptable and an operative condition determination reference value (reference value used to determine a tendency of print state) to be set in a narrower range are set in advance, and the measurement value of a pasted area is compared with these reference values.
According to the apparatus configured as above, by judging a printing state change caused by an abnormality in the printer or the like apart from the determination as to whether the product is acceptable or unacceptable, and adjusting the printer in response to such a change, it is expected that the occurrence of an unacceptable product can be prevented in advance. Further, in order to deal with an error in the area of openings of the mask sheet or the like, the apparatus of the cited reference is configured to check a measurement value distribution on the basis of measurements conducted many times and to change the operative condition determination reference according to the distribution.
Repetitive performance of printing operation on printed circuit boards by the printer makes a considerable amount of paste adhere onto the mask sheet, so that clogging or the like readily occurs. Such being the case, the mask sheet is usually cleaned at adequate intervals. According to the examination by the inventor, an amount of paste printed on a printed wiring board on which components are to be mounted is likely to differ between a time immediately after cleaning of the mask sheet and the other time even when no abnormality occurs in the printer. To be more specific, the amount of paste tends to decrease immediately after cleaning.
In a case where a determination reference to check an operative condition change caused by an abnormality in the printer or the like is set, if the printer is managed according to the same determination reference, the determination reference is required to have a normal range for the time immediately after cleaning and a normal range for the other time. This, however, raises a problem of lowering the reliability in detection of an abnormality in the printer. Even if the operative condition determination reference is changed according to the distribution of measurement values as is disclosed in the cited reference, the operative condition determination reference is adjusted merely according to the distribution of measurement values falling in a wide range including the time immediately after cleaning and the other time. It is therefore difficult to make a suitable determination for the time immediately after cleaning and the other time.